Conventionally, a heat roller fixing method has been generally used in a fixing apparatus incorporated into an image forming apparatus using an electrophotographic process. A copying machine and a printer are examples of such image forming apparatus. In the heat roller fixing method, a paper sheet (recording material) holding an unfixed toner image thereon passes between a heated fixing roller and a pressure roller which is press-contacted to the fixing roller so that the toner image is fused and fixed on the paper sheet.
A method that has been adopted in recent years is a method in which a plurality of heaters (heat sources, heating members) are disposed inside the fixing roller to partially heat the fixing roller. This method is adopted for the purposes of (i) reducing energy consumption of the image forming apparatus and (ii) improving life characteristics of the fixing roller in the fixing apparatus.
One example of an arrangement in which a plurality of heaters are disposed is an arrangement in which two heaters, a main heater and a sub-heater, are disposed. The main heater heats a center section of a fixing roller, and the sub-heater heats both end sections of the fixing roller. The main heater heats the center section of the fixing roller through which a small-sized paper sheet passes among paper sheets processable in an image forming apparatus. The sub-heater heats the end sections of the fixing roller through which a large-sized paper sheet passes.
In a surface temperature control of a fixing roller, the fixing roller is controlled so that a surface of the fixing roller keeps a preset fixing temperature. Generally, when a detected surface temperature of the fixing roller exceeds the preset fixing temperature, the heater is turned off (power to the heater is off). On the other hand, when the detected surface temperature become lower than the preset fixing temperature, the heater is turned on (power to the heater is on). In an arrangement in which a plurality of heaters are disposed, temperature sensors for detecting surface temperatures of the fixing roller are provided respectively in heating regions of the respective heaters of the fixing roller. On the basis of the temperatures detected by the temperature sensors, power to the heaters corresponding to the respective temperature sensors are on/off controlled.
Incidentally, in order to detect surface temperatures of the fixing roller with a high degree of accuracy, it is desirable to dispose the temperature sensors so as to be in contact with the surface of the fixing roller. However, an arrangement in which a plurality of temperature sensors are disposed so as to be in contact with the fixing roller causes the temperature sensors and the fixing roller to rub against each other. This may cause damage to the surface of the fixing roller, thus resulting in degradation in fixing quality. Especially, an arrangement in which the temperature sensors are disposed so as to press-contact the fixing roller tends to be a factor responsible for damage to the surface of the fixing roller. This is because pressure of the temperature sensors to the fixing roller is increased by thermal expansion that occurs in the fixing roller when the fixing roller reaches a preset fixing temperature.
In order to avoid such a problem, a temperature sensor which detects a temperature of the center section of the fixing roller (heating region of the main heater) where paper sheets from small-size paper sheets to large-size paper sheets pass (hereinafter referred to as center section temperature sensor) is generally disposed so as not to be in contact with the surface of the fixing roller. Meanwhile, a temperature sensor which detects a temperature of an end section of the fixing roller (heating region of the sub-heater) where small-size paper sheets do not pass (hereinafter referred to as end section temperature sensor) is disposed outside a maximum passing region so as to be in contact with the surface of the fixing roller. The maximum passing region is a region of the fixing roller where maximum-size paper sheets that can be used in the foregoing image forming apparatus pass.
For example, Japanese Unexamined Patent Publication No. 186909/1998 (Tokukaihei 10-186909) (published on Jul. 14, 1998) discloses an arrangement to avoid fixing failure in continuous printing. This arrangement is such that a heater is turned on during a time when a space between a recording material and a subsequent recording material passes in such a manner that a preset fixing temperature is increased as a temperature detected by a thermistor before energization of the heater is low, or such that the heater is turned on and a set temperature is increased as the amount of temperature rise is small (a speed of temperature rise is slow).
However, when temperatures of the fixing roller are controlled by using the center section temperature sensor and the end section temperature sensor disposed in the above manner, fixing failure can occur in continuous printing depending upon a paper size used.
This problem is caused by control of energization of the sub-heater on the basis of a result of detection carried out by the end section temperature sensor. In other words, the end section temperature sensor is disposed outside the maximum-size sheet passing region, and does not directly detect a temperature of the region where a paper sheet actually has passed. Because of this, a surface temperature of the sheet passing region is estimated on the basis of a temperature detected by the end section temperature sensor so that energization of the sub-heater is controlled. Practically, a relationship between a temperature detected by the end section temperature sensor and a temperature of the sheet passing region at the end section of the fixing roller is determined empirically, and a control temperature is managed on the basis of a temperature detected by the end section temperature sensor so that the temperature of the sheet passing region becomes a preset fixing temperature.
However, it is difficult to perform control so that a temperature of the sheet passing region becomes a preset fixing temperature by the temperature sensor which is disposed outside the maximum-size sheet passing region.
FIG. 9 illustrates progressions of surface temperatures of the fixing roller in (i) a region of the fixing roller where a paper sheet passes (hereinafter referred to as “passing region”) and (ii) a region of the fixing roller where a paper sheet does not pass (hereinafter referred to as “non-passing region”), in continuous printing process. FIG. 9 shows a case where one heater is provided inside the fixing roller.
In a standby mode, the surface of the fixing roller is maintained at a preset fixing temperature under on/off control of the heater. When printing process is started in this state, a surface temperature of the passing region begins to decrease because heat is taken away from the passing region by the paper sheet, and then decreases to a temperature close to a lower-limit fixing temperature. Since the heater is turned on when the surface temperature of the passing region become lower than the preset fixing temperature, there is loss of time of thermal propagation inside the fixing roller. However, the surface temperature of the passing region begins to rise and returned to the preset fixing temperature. Thereafter, the surface of the passing region is maintained at the preset fixing temperature. When the printing completes, the surface of the passing region is maintained at the preset fixing temperature under control of temperature of the fixing roller in the standby mode.
On the other hand, a surface temperature of the non-passing region slightly decreases after the printing process is started, because the fixing roller is rotated. However, heat is not taken away from the non-passing region by the paper sheet. Then, the surface temperature of the non-passing region rises quickly because the heater is turned on. Thereafter, the surface temperature of the non-passing region rises gradually, and the surface of the non-passing region is maintained at a temperature close to an upper-limit fixing temperature that is an upper limit. Thereafter, the surface temperature of the non-passing region begins to decrease because the heater is turned off at the end of printing, and the surface of the non-passing region is maintained at the preset fixing temperature under temperature control in the standby mode.
As is clear from FIG. 9, difference in temperature between the passing region and the non-passing region varies between at the beginning of the continuous printing process and at the end of the continuous printing process. Thus, when the sub-heater is on/off controlled in accordance with a temperature detected by the end section temperature sensor which is disposed outside the maximum-size sheet passing region, the event can occur where the sub-heater is not turned on in a situation where the sub-heater must be turned on in carrying out printing using large-size paper sheets.
This is explained with reference to FIGS. 10(a) through 10(d). FIGS. 10(a) through 10(d) illustrate relationships between positions of the fixing roller in its axial direction and surface temperatures of the fixing roller, just before the start of continuous printing, right after the start of the continuous printing using large-size paper sheets, in the middle of the continuous printing, and at the end of the continuous printing, respectively.
As illustrated in FIG. 10(a), a surface temperature of the fixing roller just before the start of continuous printing is constant in the entire region of the fixing roller in its axial direction. In such a state, when the continuous printing is started and large-size paper sheets pass, a temperature of the sheet passing region becomes lower than the preset fixing temperature. This causes a temperature detected by the center section temperature sensor to become lower than a control temperature, and the main heater is therefore turned on. Meanwhile, a temperature detected by the end section temperature sensor decreases because heat is taken away from the end section of the fixing roller by the sheet passing region whose temperature has decreased. Thus, the sub-heater is also turned on.
Right after the start of the continuous printing, as illustrated in FIG. 10(b), a surface temperature of the sheet passing region is constant in the entire region of the fixing roller in its axial direction because both the main heater and the sub-heater are turned on. Note that a surface temperature of the sheet passing region is slightly lower than the preset fixing temperature due to loss of thermal expansion inside the fixing roller and other reasons. A region where paper sheets do not pass in the end section of the fixing roller (hereinafter referred to as non-sheet-passing region) is heated under heat of the sub-heater which is turned on. Accordingly, a surface temperature of the non-sheet-passing region exceeds the preset fixing temperature. Since the end section temperature sensor is disposed in the non-sheet-passing region, difference in temperature from the sheet passing region increases. A surface temperature of the non-sheet-passing region becomes increasingly higher when the sub-heater keeps being turned on. As a result, the above-mentioned difference in temperature increases as printing is continued.
When difference in surface temperature between the sheet-passing region and the non-sheet-passing region increase, a temperature detected by the end section temperature sensor does not become lower than the control temperature. This results in the following event: Although a temperature of the sheet passing region does not actually reach the preset fixing temperature as illustrated in FIG. 10(c), the sub-heater is not turned on, and only the main heater is turned on. In addition, a temperature of the non-sheet-passing region keeps rising under heat of only the main heater which is turned on.
This event occurs due to unnecessary heat supply by the main heater. That is, in the case of heaters whose heat generating regions that give off heat are different from each other, including the main heater and the sub-heater, positions that are in no need of heating in a fixing roller are unnecessarily heated. FIG. 11 illustrates a relationship between a position of the fixing roller in its axial direction and a heat supply rate in the main heater and the sub-heater.
When the main heater keeps being turned on, such an unnecessary heat supply by the main heater causes unnecessary heat supply to the end section of the fixing roller. As a result, a surface temperature of the region where the end section temperature sensor is disposed further rises gradually because a method for dissipating heat of the region where the end section temperature sensor is disposed is just heat release in the air or thermal movement into the fixing roller by heat conduction.
As a result, at the end of the continuous printing, although the center section of the fixing roller is maintained at the preset fixing temperature, temperatures of the end sections of the fixing roller become lower than the preset fixing temperature, as illustrated in FIG. 10(d).
Due to such variations in temperature on the surface of the fixing roller, a paper sheet suffers from fixing failure such that the paper sheet has a lower gloss at a part that have passed through the center section of the fixing roller where a temperature is high than at parts that have passed through the end sections of the fixing roller where a temperature is low.
It is to be noted that the invention described in the aforesaid Japanese Unexamined Patent Publication cannot solve the above problem because it does not assume a fixing apparatus that has a plurality of heaters disposed inside a fixing roller to partially heat the fixing roller, and it does not estimate a temperature of the sheet passing region by using a temperature sensor which is disposed outside the maximum-size sheet passing region.